Known soft magnetic materials for magnetic cores or the like include metal soft magnetic materials such as Sendust and Permalloy and metal oxide soft magnetic materials such as ferrite. The metal soft magnetic materials have a high saturation magnetic flux density and high magnetic permeability, but experience great eddy current losses in a high frequency band because of low electric resistivity. They are thus difficult to use in the high frequency band. On the other hand, the metal oxide soft magnetic materials provide less eddy current losses in the high frequency band because of their higher electric resistivity than the metal soft magnetic materials. However, the metal oxide soft magnetic materials are unsatisfactory in saturation magnetic flux density.
Under such circumstances, composite soft magnetic materials having high saturation magnetic flux density and magnetic permeability as well as high electric resistivity were proposed as the soft magnetic material which overcame the drawbacks of both the metal soft magnetic material and the metal oxide soft magnetic material. For example, Japanese Patent Application Kokai (JP-A) No. 91397/1978 discloses a high magnetic permeability material comprising a metal magnetic material having a coating of high magnetic permeability metal oxide formed on the surface; JP-A 164753/1983 discloses a composite magnetic material prepared by mixing an oxide magnetic material powder and a metal magnetic material powder composed of an Fe-Ni base alloy and molding the mixture; and JP-A 13705/1989 discloses a composite magnetic material having a saturation magnetic flux density Bs of 6.5 to 20 kG, comprising a soft magnetic metal magnetic powder having a mean particle size of 1 to 5 .mu.m and a soft ferrite wherein the soft ferrite fills in between the metal magnetic powder particles so that the metal magnetic powder particles are independent from each other while the soft ferrite portion is continuous.
Prior art composite soft magnetic materials including the ones disclosed in the foregoing patent publications are fired by hot press sintering, vacuum sintering, and atmospheric pressure sintering processes like ambient sintering. The firing temperature generally ranges from about 900.degree. to about 1200.degree. C. and a firing time of one hour or longer is generally required. However, metal soft magnetic materials, when held for more than one hour at elevated temperatures, are oxidized by oxygen available from the metal oxide soft magnetic materials which are, in turn, reduced. The situation remains the same even when the materials are fired in a reducing atmosphere. Since the metal soft magnetic material and metal oxide soft magnetic material lose their own features, a composite soft magnetic material having high saturation magnetic flux density and magnetic permeability as well as high electric resistivity is no longer obtained.
The inventors proposed a composite soft magnetic material obtained by plasma activated sintering a mixture soft magnetic metal particles and a high resistance soft magnetic substance in U.S. patent application Ser. No. 07/696,911 filed May 8, 1991. More particularly, soft magnetic metal particles are coated with a high resistance soft magnetic substance and a mass of the coated particles is placed in a plasma. Then charged particles including gas ions and electrons generated by electric discharge impinge against the contact between the coated particles for cleaning the contact area. Charged particle bombardment, coupled with evaporation of the substance at the contact area, provides an intense bombardment pressure to the coated particle surface. The high resistance soft magnetic substance on particles is increased in internal energy or activated. Therefore, the sintering time is reduced, for example, sintering is completed within about 5 minutes. As a result, oxidation of soft magnetic metal particles and reduction of high resistance soft magnetic substance are avoided and there can be provided a composite soft magnetic material having a high saturation magnetic flux density, high magnetic permeability, and high electric resistivity. However, this material is still insufficient in power or core loss. There is a need for further improvement in this regard.